Field of the Invention
The invention relates generally to downhole motors used for drilling oil and gas wells. More particularly, invention relates to means for retaining a seal boot on a universal joint housing for a driveshaft of a downhole motor.
Background of the Technology
In drilling a borehole (or wellbore) into the earth, such as for the recovery of hydrocarbons or minerals from a subsurface formation, it is conventional practice to connect a drill bit onto the lower end of a “drill string”, then rotate the drill string so that the drill bit progresses downward into the earth to create the desired borehole. A typical drill string is made up from an assembly of drill pipe sections connected end-to-end, plus a “bottom hole assembly” (BHA) disposed between the lower end of the drillstring and the drill bit. The BHA is typically made up of sub-components such as drill collars, stabilizers, reamers and/or other drilling tools and accessories, selected to suit the particular requirements of the well being drilled.
In borehole drilling operations, the drill string and bit are rotated by means of either a “rotary table” or a “top drive” associated with a drilling rig erected at the ground surface over the borehole (or in offshore drilling operations, on a seabed-supported drilling platform or suitably-adapted floating vessel). During the drilling process, a drilling fluid, commonly referred to as “drilling mud,” “drilling fluid,” or simply “mud”, is pumped under pressure downward from the surface through the drill string, out nozzles in the face of the drill bit into the wellbore, and then upward back to the surface through the annulus between the drill string and the wellbore. The drilling fluid carries borehole cuttings to the surface, cools the drill bit, and forms a protective cake on the borehole wall to help stabilize and seal the borehole wall, as well as other beneficial functions. At surface the drilling fluid is treated, by removing borehole cuttings, amongst other possible treatments, then re-circulated by pumping it back downhole under pressure through the drill string.
As an alternative to rotation by a rotary table or top drive alone, a drill bit can also be rotated using a “downhole motor” incorporated into the drill string immediately above the drill bit. The technique of drilling by rotating the drill bit with a downhole motor without rotating the drill string is commonly referred to as “slide” drilling. It is common in certain types of well-drilling operations to use both slide drilling and drill string rotation, at different stages of the operation. The use of downhole motors has generally increased in recent years due, at least in part, to their employment in the drilling of wellbores directionally, since downhole motors provide some advantages in such applications.
The downhole motor, which may also be referred to as a mud motor or progressive displacement motor (PDM), converts hydraulic energy of a fluid such as drilling mud into mechanical energy in the form of rotational speed and torque output, which may be harnessed for a variety of applications such as downhole drilling. A typical downhole motor includes a hydraulic drive section, a drive shaft assembly, and a bearing assembly. The hydraulic drive section, also known as a power section or rotor-stator assembly, includes a helical rotor rotatably disposed within a stator, the driveshaft assembly includes a driveshaft rotatably disposed within a driveshaft housing, and the bearing assembly includes a mandrel rotatably supported within a housing. The lower end of the rotor is connected to the upper end of the driveshaft, the lower end of the driveshaft is connected to the upper end of the mandrel, and the lower end of the mandrel is coupled to a drill bit. During drilling operations, the high pressure drilling fluid is pumped under pressure down the drillstring and between the rotor and stator, causing the rotor to rotate relative to the stator. Rotation of the rotor drives the rotation of the driveshaft, the mandrel, and the drill bit.
The driveshaft assembly generally functions to transfer torque from the eccentrically-rotating rotor to the concentrically-rotating bearing assembly mandrel and drill bit. The motor may also be configured such that the longitudinal axis of the power section is skewed or oriented at an acute angle relative to the longitudinal axis of the bearing section and drill bit. The driveshaft of the driveshaft assembly typically includes universal joints at its upper and lower ends to accommodate the misalignment of axes during operation while allowing transfer of torque from the power section to the bearing assembly and drill bit. The universal joints are usually axially received within upper and lower adapters. The upper adapter connects to the rotor of the power section, and the lower adapter connects to the mandrel of the bearing assembly. A threaded end cap or sleeve is typically employed at each universal joint to retain an elastomeric seal boot that encases and seals the components of the universal joint from drilling mud, and maintains the position of the universal joints and driveshaft ends within the driveshaft adapters.
During downhole operations, it is critical that the end caps remain threadably secured to the adapters. Specifically, if an end cap loosens and disengages from an adapter, (a) the components of the universal joint may separate from the adapter, potentially disabling the ability to transfer torque from the power section to the drill bit, and further, (b) the seal boot will cease to function and components of the universal joint will become undesirably exposed to the drilling fluid, potentially leading to universal joint failure.
In most conventional driveshaft assemblies, application of a thread-locking compound to the mating threads of the end caps and adaptors, followed by the application of makeup torque to the threaded end caps is employed to prevent unthreading and loosening of the end caps from the adapters. Due to the relatively short axial length of typical end caps, the typical means to apply the necessary makeup torque is through the use of a pipe wrench connected to an overhead crane with a load cell that measures the torque applied to the end cap as the connection is made up. This approach occasionally fails, and further, introduces safety hazards and difficulties during servicing. For example, the pipe wrench is susceptible to fracturing under the excessive torque loads required in some cases, potentially causing pieces of the pipe wrench to be dangerously projected across the shop floor at high velocities. Moreover, the pinching loads applied to the end caps as the pipe wrench bites into the cap may overstress and/or deform the cap. Thread-locking compound also increases the difficulty in servicing the driveshaft assembly, particularly during disassembly, where the end cap-adapter threaded connection must first be heated to liquefy the hardened thread-locking compound to permit subsequent unthreading of the end cap and the adapter. Still further, even with the use of thread-locking compound, high vibrations and harsh downhole conditions can occasionally lead to unthreading of the caps and adapters.
Accordingly, there remains a need in the art for devices, systems, and methods for securing an end cap to a driveshaft assembly adapter. Such devices, systems, and methods would be particularly well received if they restricted and/or prevented disengagement of the end cap and the adapter during downhole operations, maintained the position of the protective seal boot, and were relatively cheap and simple to employ.